1. Field of the Invention
The present invention relates to an identification method and system which employ a multiple layer machine readable identification label as well as to the label itself, and in particular, to an identification method and system which employ a multiple layer machine readable label having multiple marking layers each encoding a respective identification symbol readable using one or a plurality of sensing technologies to detect the symbol encoded therein.
2. Background of the Invention
Private industries and government agencies such as automotive, communication, distribution, manufacturing, medical/dental, nuclear, pharmaceutical, printing/publishing, security, aerospace/aviation, and defense have a need to apply markings (e.g., labels) to products for esthetic and security reasons.
Conventional labels use single layer identification symbols such as a UPC barcodes (one-dimensional symbols) and more recently two-dimensional symbols (e.g. two-dimensional symbols named by the automated identification and data collection (AIDC) industry). Two-dimensional symbols may be matrix or stacked barcodes that form an encoded array. U.S. Pat. Nos. 4,939,354 and 5,053,609 to Priddy et al. disclose a matrix code (Data Matrix Symbol) designed as a label to be applied directly to products. This matrix code can store from one to 2335 alphanumeric characters in any language.
FIG. 1 illustrates the basic elements of a matrix symbol known in the art. Although shown as a square, matrix symbols may be of any shape. However, square and rectangular shapes are most commonly used in the industry. The unshaded squares comprising the matrix shown in FIG. 1 each represent a binary “0” and the shaded squares each represent a binary “1”. FIG. 2 illustrates an example of a data matrix symbol as it would appear on a product to be marked.
In specialized circumstances, it is advantageous to have the label be invisible (i.e., not viewable to the naked eye). Currently, this may be accomplished using inks that when applied to a visible surface, are invisible to the unaided eye. These inks are produced by adding special materials to a carrier ink. Using an appropriate sensor, the marking can be detected.
One disadvantage with current invisible ink materials is that these materials degrade over time. For example, these materials are adversely affected by sunlight.
A second disadvantage with current identification markings is that they are adapted to be optically detected. Consequently, these markings cannot be read if covered by an opaque layer such as paint. Further, only a single layer identification symbol can be used as a label to identify a product because only a single (i.e., top) layer is detectable.